1. Field of the Invention
The present invention relates to a semiconductor device containing a circuit structured by a thin film transistor (hereinafter referred to as a TFT), and to a method of manufacturing thereof. For example, the present invention relates to an electro-optical device, typically a liquid crystal display panel, and to electronic equipment (electronic instruments) loaded with this type of electro-optical device as a part.
Note that, throughout this specification, the term semiconductor device indicates general devices which function by utilizing semiconductor characteristics, and that electro-optical devices, semiconductor circuits, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
Techniques of structuring a thin film transistor (TFT) using a semiconductor thin film (having a thickness on the order of several nm to several hundreds of nm) formed on a substrate having an insulating surface have been in the spotlight in recent years. The thin film transistor is widely applied in electronic devices such as ICs and electro-optical devices, and in particular, its development is accelerating as a switching element of a liquid crystal display device.
Active matrix liquid crystal display devices which use TFTs as switching elements for connecting to pixel electrodes arranged in a matrix shape are gathering attention for use in obtaining a high quality image in a liquid crystal display device.
Active matrix liquid crystal display devices are roughly divided into two types, a transmitting type and a reflecting type.
In particular, the reflecting type liquid crystal display device has the advantage of low energy consumption compared to the transmitting type liquid crystal display device because it does not use a backlight, and its demand in direct view displays for mobile computers and video cameras is high.
Note that the reflecting type liquid crystal display device utilizes the optical modulation action of a liquid crystal, and a state of outputting incident light, which is reflected by pixel electrodes, to the outside of the device and a state of not outputting incident light to the outside of the device are selected, performing bright and dark display. In addition, display of an image is performed by combining these two states. The pixel electrodes in a reflecting type liquid crystal display device are generally composed of a metallic material having a high light reflectivity such as aluminum, and these are electrically connected to switching elements such as thin film transistors.
Gate wirings (scanning lines), source wirings (signal lines), and capacitor wirings are each patterned into a linear shape with the pixel structure of a conventional reflecting type liquid crystal display device. Further, the source wirings are arranged in a horizontal direction, the gate wirings are arranged in a vertical direction, and interlayer insulating films are formed between the gate wirings and the source wirings in order to insulate the wirings. In addition, in a conventional structure, a portion of the source wirings and a portion of the gate wirings intersect, and TFTs are arranged in the vicinity of the intersecting portions.
Furthermore, an additional interlayer insulating film is formed on the source wirings conventionally, and the pixel electrodes are formed on this interlayer insulating film. The number of steps increases when the number of layers increases with this structure, and this invites an increase in costs.
A structure in which pixel electrodes are formed between source wirings at the same time the source wirings are formed is known as another conventional structure. In this case, it is necessary to perform shielding by using a black matrix between the source wirings and the pixel electrodes.
Shielding of a TFT and shielding between pixels are conventionally performed in accordance with a black matrix, in which a metallic film formed of a material such as chrome is patterned into a desired shape. However, in order to have sufficient shielding of light by the black matrix, it is necessary to insulate by forming an interlayer insulating film between the black matrix and the pixel electrodes. If the number of layers of interlayer insulating films thus increases, the number of steps increases, inviting increased costs. Further, it is disadvantageous to have interlayer insulating properties. In addition, the number of steps for forming the black matrix itself and the number of masks are increased.
Seen from the viewpoint of display performance, a storage capacitor and a high aperture ratio are required for pixels. By giving each pixel a high aperture ratio, the efficiency of light usage increases, and the display device can be made energy efficient and small in size.
The reduction in the size of pixels has been advancing in recent years, and higher definition images are demanded. The reduction in pixel size means that the amount of surface area occupied for forming a TFT and wirings for each pixel becomes larger, and the aperture ratio of the pixels decreases.
In order to obtain a high aperture ratio in each pixel within standard size pixels, it is indispensable to layout the circuit elements required in the circuit structure of the pixel very efficiently.